Insects - Pests of Ornamental Trees, Shrubs and Flowers - page 159

Scolytus scolytus (Fabricius)
Large elm bark beetle
An important pest of elm ( Ulmus ), the beetles acting as
vectors of Dutch elm disease (a disease caused by
fungi in the genus Ophiostoma ); dying or at least
weakened ash ( Fraxinus excelsior ), poplar ( Populus ),
oak ( Quercus ) and various other debilitated trees are
also attacked. Widely distributed in Europe.

to the eventual death of branches or complete trees, are
usually initiated only in unhealthy hosts or in those
under severe root stress.

Scolytus mali (Bechstein)
Large fruit bark beetle
This widespread species is found occasionally on
ornamentals such as Chaenomeles , Cotoneaster , crab-
apple ( Malus ), flowering cherry ( Prunus ), hawthorn
( Crataegus ) and Sorbus , but is more commonly
associated with rosaceous fruit trees. Breeding colonies
usually occur in the trunks and larger branches of
already weak or dying host plants. Each colony consists
of a long maternal chamber and a series of about 50-60
more or less perpendicular larval galleries. These spread
upwards and downwards, immediately beneath the bark,
each terminating in a slight bulb in which pupation
occurs.

DESCRIPTION
Adult: 5-6 mm long; mainly black with reddish-
brown elytra; front tibiae toothed; a median peg-like
tubercle on the hind margin of the third and fourth
sternites. Larva: white, C-shaped and legless; head
dark brown.

LIFE HISTORY
Young adult beetles appear from mid-May to October,
congregating at the tops of elm trees to feed on the
small twigs. Beetle activity is greatest during warm
weather and, if carrying Dutch elm disease spores on
their mouthparts or bodies, the insects readily introduce
the fungus into healthy tissue. Infections in May or
June are most important, the spores usually gaining
entry through the feeding channels formed by the
beetles in the crotches between the twigs. Later in the
summer, the beetles burrow into the trunks and
branches of weak or dying hosts to form short (25 mm
long) perpendicular breeding chambers immediately
beneath the bark. After mating, each female lays about
50 eggs along the length of her chamber. The eggs
hatch about ten days later. Larvae then burrow away
from the maternal chamber, between the bark and sap
wood, to produce a series of feeding galleries. These
galleries form an irregular, fan-like pattern, on either
side of the main chamber, which is characteristic for
the species and clearly visible when the covering of
dead bark is peeled away. The larvae are fully grown by
the winter or following spring, their rate of
development varying considerably from site to site.
They then pupate, each in a slight bulb at the end of its
burrow. Adults eventually emerge via small, rounded
flight holes which they excavate through the bark. Host
trees remain suitable as breeding sites for bark beetles
and, hence, as potential reservoirs of Dutch elm disease
for about two years after death.

Scolytus multistriatus (Marsham)
Small elm bark beetle
A relatively small species (2.0-3.5 mm long),
associated mainly with elm ( Ulmus ) but also capable of
damaging other trees such as oak ( Quercus ) and
poplar ( Populus ). The adults are structurally similar to
those of Scolytus scolytus , the other elm-feeding
species, but have a pair of lateral teeth on the hind
margin of the second, third and fourth abdominal
sternites. The biology of both species is similar, and
both act as vectors of Dutch elm disease; the larval
galleries of S. multistriatus are slightly smaller than
those of S. scolytus , and characterized by their more
regular appearance.

Scolytus rugulosus (Müller)
Fruit bark beetle
Although most commonly associated with fruit trees,
especially cherry and plum, infestations of this
widespread species also occur on flowering cherry
( Prunus ) and some other rosaceous ornamentals.
Attacks typically occur in the trunks of small trees and
in branches up to 6 cm in diameter. The larval galleries,
which lead away from the maternal chamber between
the bark and sap wood, commonly overlap, particularly
towards their extremities.

DAMAGE
Elm: infestations cause the die-back of branches and
hasten the decline of host trees. If Dutch elm disease
takes a hold, the leaves of infected branches suddenly
turn yellow, the fungus usually spreading and eventually
killing the whole tree. Other hosts: attacks, which lead